The use of phase modulation in recording the time constant for the decay of the photons exiting f rom human organs such as brain and limbs has led to the development of several successful devices, in which precise control of oscillator frequencies has resulted in photon migration measurements accurate to a few millimeters using frequencies of 220 MHz. Recent improvements in sensors employing silicon diode detectors suggest that frequencies approaching 500 MHz can be used. At this frequency the amplitude modulation index of the emergent photons depends upon the path length as, for example, in the attenuation characteristic of a parallel RC network. Whereas the phase characteristic Of such an RC network for tissue studies has been previously emphasized, in this case we propose a novel development in which the amplitude charaCteristic of the photon migration is studied. In this grant application we propose to evaluate measurement of the amplitude index at 440 MHz with 140 MHz as a reference frequency. The proposed instrument measures the ratio (or difference) of the amplitude modulation index at a low and a high frequency and at two wavelengths. The attenuation characteristic at the two frequencies and two wavelengths becomes, a measure of path length changes. Demonstrations in a model system and in human tissues at 97 and 220 MHz ensure the feasibility of the proposal.